This invention relates to pulse signalling systems of the code type and more particularly to an improved autocorrelation technique for use in such pulse signalling systems.
Correlation techniques have been utilized in the past in signal processing systems employing signals in the form of a pulse or sequence of pulses. Such pulse signalling systems include, for example, radiant energy reflecting systems, such as radar, radio range finders, radio altimeters, and the like; pulse communication systems, such as over-the-horizon systems employing various types of scatter techniques, satellite communication systems and the like; and multiple access systems employing address codes to enable utilization of the multiple access system. Correlation techniques when employed in coded radiant energy reflection systems enhance the resolution of closely spaced reflecting surfaces and in addition, increase the average power transmitted. Correlation techniques employed in pulse communication systems result in increased signal-to-noise ratios without increase of transmitter power and minimize multiple paths affects (fading). Correlation techniques when employed in a multiple access environment also result in increased signal-to-noise ratio without increase of transmitter power and if properly coded prevents or at least minimizes the interference or crosstalk between one or more address codes.
According to prior art correlation techniques the received signal is processed by obtaining the product of code elements of the received signal and code elements of a locally generated signal of the same waveform and period as the received signal and integrating the resultant product. The optimum output for such a correlation would be a single peak of high amplitude which has a width substantially narrower than the pulse width of the received signal. Most correlation systems in use today do not produce the desired optimum waveform, but rather provide an output whose waveform has spurious peaks in addition to the desired high amplitude peak. The presence of these spurious peaks is undesirable in that the resolving power of radiant energy reflecting system is reduced, the signal-to-noise ratio of pulse communication systems and multiple access systems and the minimization of multiple path affects of pulse communication systems is reduced to a level below the optimum value.
Previously a number of improved correlation technique have been proposed that will result in an impulse correlation function. The term "impulse correlation function", and more specifically, "impulse autocorrelation function", as employed herein, refers to a waveform having a single high amplitude peak completely free from spurious peaks of lower amplitude elsewhere in the waveform.
One of the proposed improved correlation techniques which is related to the present invention is fully disclosed in the copending application of F. S. Gutleber, Ser. No. 645,697, filed June 13, 1967, (hereinafter referred to as "said first copending application"). Said first copending application discloses a class of codes including two codes, termed code mates, where the code mates have cooperating autocorrelation functions so that when they are autocorrelation detected and the resultant detected outputs are linearly added there is provided an impulse autocorrelation function having an impulse output at a given time and a zero output at all other times. The code mates generated are time or frequency multiplexed for transmission to the detector to provide long code sequences to increase the average transmitting power. The transmitted multiplex code mates are separated consistent with the type of multiplexing being employed prior to autocorrelation detection and linear addition. The number of code mates can be increased in accordance with said first copending application by interleaving each of the original code mates and the different time displaced versions thereof to provide a plurality of first codes and interleaving one code mate and the complemented version of the other code mate of each of the original code mates and the different time displaced versions thereof to provide a code mate for each of the first codes. This process of increasing the number and length of code mates can be continued repeatedly with the newly generated codes mates to further increase the number and length of code mates.
Another of the proposed improved autocorrelation techniques which is related to the present invention is fully disclosed in the copending application of F. S. Gutleber, Ser. No. 671,382, filed Sept. 28, 1967, (referred to hereinafter as "said second copending application"). Said second copending application discloses another class of codes employing code mates having cooperating autocorrelation functions so that when they are detected and the resultant detected outputs are linearly added together there is provided the desired impulse autocorrelation function. As in said first copending application the code mates generated are time or frequency mutliplexed for transmission to the detector to provide long code sequences to increase the average transmitting power. Unlike said first copending application, the code mates of said second copending application do not require complete fill-in but rather may include blank digit time slots of any given number at the end of a code prior to repetition thereof, or the blank slots may be disposed between the first and last time slots which are required to have a digit of either of two conditions, such as binary "1" or "0". The number and length of code mates can be increased in accordance with said second copending application by employing the interleaving processes called for in the equations of FIG. 5 thereof. Each of the new code mates thusly generated may be processed according to the equations of FIG. 5 to form still other new code mates. This process may be continued repeatedly for each new code mate generated to further increase the number and length of code mates.
Still another proposed improved correlation technique which is related to the present invention is fully disclosed in the copending application of F. S. Gutleber, Ser. No. 669,899, filed Sept. 22, 1967, (referred to hereinafter as "said third copending application") said third copending application discloses still another arrangement utilizing the code mates of said first and second copending applications to expand the number of code mates and the length of the code mates. The number and length of code mates can be increased in accordance with said third copending application by employing the butting process called for in the equations of FIG. 1 thereof. Each of the new code mates thusly generated may be processed according to the equations of FIG. 1 to form still other new code mates. This process may be continued repeatedly for each new code mate generated to further increase the number and length of code mates.